Is it right? No. People should not take things that do not belong to them. We learn that in pre-school. Do they do it anyway? Yes. Is it fairly common knowledge people steal things? yes. Has the OP had this happen to him in the past? Yes. So, despite it not being right, we know it happens, it has happened, and people still leave their stuff laying behind when they venture out for the day anyway. The only obvious option is to mitigate the risk by not leaving your stuff lay around. Otherwise, one can come on here and complain all day, it does not change the fact your stuff was stolen. The only way to stop that is don't leave it in a place for it to be stolen. Not sure why this thread got more complicated than that. Sure it's bad, but you have the power to prevent it.
When I was younger my parents bought me a bicycle. It was a very cool bike. Schwinn crate 3 speed. We lived in an apartment on the ground floor with a patio. My parents told me to bring it in to the locked inside storage area over night. It was a hassle. So I started just leaving it on the patio. Well it got stolen. Sure it was our property. Sure it was illegal. Didn't change the fact that my bike was stolen. My parents scolded me for leaving it in a place they specifically told me not to. And told me that despite the thief stealing it, I was partly responsible. Or at least irresponsible. The next bike I paid for with my allowance. I kept it locked up inside and never had an issue.
So complain all you want, but leave your stuff in the open unattended and you are rolling the dice.
BTW - I have never had anything stolen at a CG. Ever. I lock stuff up at night in bins, bikes on the carrier, locked, and I keep it that way when venturing out for the day. Hassle? Maybe - depends on your perspective. Getting stuff stolen is a bigger hassle.